A role for self-gravity at multiple length scales in the process of star formation

Self-gravity plays a decisive role in the final stages of star formation, where dense cores (size similar to 0.1 parsecs) inside molecular clouds collapse to form star- plus- disk systems(1). But self- gravity's role at earlier times ( and on larger length scales, such as similar to 1 parsec) is unclear; some molecular cloud simulations that do not include self- gravity suggest that 'turbulent fragmentation' alone is sufficient to create a mass distribution of dense cores that resembles, and sets, the stellar initial mass function(2). Here we report a 'dendrogram' (hierarchical tree- diagram) analysis that reveals that self- gravity plays a significant role over the full range of possible scales traced by (CO)-C-13 observations in the L1448 molecular cloud, but not everywhere in the observed region. In particular, more than 90 per cent of the compact 'pre- stellar cores' traced by peaks of dust emission(3) are projected on the sky within one of the dendrogram's self- gravitating 'leaves'. As these peaks mark the locations of already- forming stars, or of those probably about to form, a self- gravitating cocoon seems a critical condition for their existence. Turbulent fragmentation simulations without self- gravity even of unmagnetized isothermal material - can yield mass and velocity power spectra very similar to what is observed in clouds like L1448. But a dendrogram of such a simulation(4) shows that nearly all the gas in it ( much more than in the observations) appears to be self- gravitating. A potentially significant role for gravity in ` non- self- gravitating' simulations suggests inconsistency in simulation assumptions and output, and that it is necessary to include self- gravity in any realistic simulation of the star- formation process on subparsec scales.